We have previously demonstrated that prolactin (PRL) can stimulate the function of the testis and that this is due to increasing the responsiveness of the testis to LH stimulation. In more recent work, supported by this award, we have demonstrated that PRL has a major role in mediating the effects of changes in the photoperiod on testicular activity in the golden hamster, a seasonally-breeding species. We have also shown that PRL increases the concentration of LH receptors in the hamster testis. In another series of experiments, we have demonstrated that excessive PRL release does not lead to reduced testicular activity in several species of laboratory animals, but it can reduce the concentration of gonadotropins in the plasma and suppress male copulatory behavior. It is proposed to continue our studies on the physiological role of PRL in the regulation of testicular activity. We will determine whether major shifts in testicular activity and in plasma PRL levels in hamsters exposed to different photoperiods are accompanied by changes in testicular PRL receptors. We will also try to define the hormonal regulation of PRL receptor levels in the testis under various conditions. Furthermore, we will determine whether suppression of PRL release and artificial maintenance of high PRL levels can affect the course of spontaneous testicular recrudescence in hamsters maintained in short photoperiod. We will also examine the patterns of LH, FSH and testosterone levels associated with recrudescence in animals with normal, suppressed and elevated PRL release. Finally, we will determine to what extent the effects of PRL on the testis may be mediated by the apparent ability of PRL to stimulate FSH release. These experiments will involve hamsters maintained in different photoperiods, mice with hereditary PRL deficiency, and genetically normal immature rats and mice. The results should advance the understanding of pituitary control of the testis in health and disease.